Use the Right Tool

While doing some construction work recently, I needed a tool that was in my truck, but didn’t feel like going to get it. So I tried to “make do” with what I had in my toolbox. Needless to say, the job took twice as long and didn’t come out the way that I wanted. So I ended up going to get the right tool anyway.

Three common power-quality tools are the screwdriver, the DVM and the dedicated power-quality analyzer. Each one has its value and place in efficiently taking care of power-quality problems.

One of the fundamental tools for power quality is the screwdriver. Even this simple tool requires the use of proper protective equipment and following the proper safety procedures when dealing with potentially lethal voltages and/or arc-flash hazards. The screwdriver is handy for safely taking off the distribution panels. It’s also good for making a visual inspection for improper neutral-to-ground bonds and the lack of continuity in the equipment-grounding conductor and/or the grounded conductor (the latter often referred to as the neutral conductor on three-phase, four-wire wye circuits).

According to the NEC, the grounding and grounded conductors should only be bonded in one place, at the service entrance, unless there is a separately derived source. Failure to adhere to this can result in a possible life-safety hazard as well as improper equipment operation from ground loops and elevated ground potentials. In addition, the screwdriver is a useful tool for tightening some types of loose connections, such as the wires to low-voltage breakers in such panels, which can be the cause of intermittent interruptions. Sometimes these interruptions can be correlated to vibration from equipment in the facility or even heavy vehicular traffic outside the facility. But often, the equipment just shuts down without even a light blink being observed.

The DVM is another useful tool in the power-quality arena when used within its capabilities. Voltage unbalance (or imbalance, depending on your preference) of a polyphase circuit can usually be determined by a few quick readings. As the NEMA MG-1 standard points out, just a three-percent voltage imbalance can result in the need to derate electric motors by 10 percent. Some handheld meters have the ability to measure harmonics, another of the evils for motors and transformers. Both unbalance and harmonics are considered as steady-state problems, since the primary effect is heating through extra losses in the equipment. Heating takes time to build up, so an occasional high harmonic content isn’t going to cause this type of problem, though it could cause others, such as multiple zero crossings that confuse clock circuits.

Short duration disturbances, such as sags, swells and transients, aren’t often detectable by DVM-type meters. Some of these only update the readings once every quarter of a second or so. Since the majority of sags last less than 10 cycles (1/6 of a second), you would have to be pretty lucky to see one on the display, and it would be gone again with the next reading. In addition, the sag could occur on one phase and not the other. Or, the sag on one phase could result in swells on the other two phases. Depending which conductor you were measuring at the time, you might be tricked into misdiagnosing the problem.

And then there are the dedicated power-quality analyzers. Most of these instruments are designed as monitors to be left at the location for days or weeks at a time and to trigger on those only-when-this-happens-and-this-happens-while-this-is-going-on-type problems that only occur once every couple of weeks but shut down the entire production line when they do. Depending on the instrument, they can capture many problems, from sub-microsecond transients to sustained interruptions lasting an hour or more. They can provide graphical displays of wave forms and trending information of hundreds of parameters to look for clues to the cause of such problems. With built-in oscilloscope, phasor diagrams, and real-time metering, they can provide information simultaneously on all three phase conductors, along with neutral-to-ground voltage and associated currents. This presents a more complete picture of what is going on in the electrical system, but at a price considerably more than a DVM. Until the need grows, renting may be a more affordable way to add such capabilities to your toolbox.

Most of the power-quality analyzers have accompanying application software that runs on a PC. These software tools are useful for doing more in-depth analysis of the data collected in the instrument, comparing one monitoring location to another, looking for trends at the same site during the same month in a previous year or writing a report to document to the customer what is going on in their facility. There are even programs that will attempt to determine the cause and location of the disturbance, such as when that squiggly voltage waveform is the result of an upstream power-factor capacitor switching event that resulted in a transient that tripped the overcurrent protection on an adjustable-speed drive.

Perhaps the best scenario is the preventative one—finding out a potential problem before it shuts the system down. The right tool for the right job can save hours of work and thousands of dollars in potential lost production from a power-quality disturbance. Using the “most convenient” tool can result in missing the real problem and lost time for both you and your customer. EC

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.